It is absolutely not necessary to place a messaging provider (Broker, JMS, MQ Series, etc) between two Integration Servers in order for them to communicate. IS can use built-in service pub.remote:invoke to invoke a service on any IS that has been configured in the remote servers settings. In addition, one IS can post an XML doc to another over HTTP/S, consume a web service exposed by the other IS, send it email or FTP it a file. There are so many ways to accomplish this without messaging in the middle.
But perhaps the organization has standardized on Websphere MQ Series and wants to force it between every nook and cranny of the enterprise architecture at the bequest of their friendly IBM sales rep.
Regardless of the motivation, you need to obtain a license for the webMethods MQ Series adapter for each IS, configure an adapter connection to an MQ Series server and then develop message handling services to allow one IS to send messages to the second IS and vice versa. The webMethods MQ Series Adapter Users Guide covers how to use it’s adapter services to send and receive messages as well as how to create an adapter notification to monitor an MQ Queue to listen for messages.
In response to an inquiry I sent last week, WM Support confirmed that the webMethods MQ Adapter now supports Websphere MQ 6.x although this is not stated in the User’s Guide.